With 5-10 cases of epilepsy per 1000 people in the world, and temporal lobe epilepsy making up over 66% of these cases, temporal lobe epilepsy is considered one of the most prevalent and serious neurological disorders. The annual economic burden of over $12.5 billion in the US alone adds to the considerable cost in the quality of life of epileptic patients, and calls for efforts to better understand the mechanisms underlying spontaneous seizures. A neuronal cell population that may play a key role in seizure activity is hippocampal mossy cells, which are critically important in the dentate gyrus excitatory network. This project proposes to test the hypothesis that mossy cells are central to amplifying and propagating hyperexcitable activity, and contribute to the development of debilitating epilepsy-related cognitive deficits. The hypothesis will be tested during the chronic epilepsy period of the intrahippocampal kainate epilepsy model. Alterations occurring in the dentate gyrus of the mouse hippocampus will be investigated using a combination of in vivo electrophysiological and closed-loop optogenetic methods, as well as learning and memory studies. Discerning the role of mossy cells in temporal lobe epilepsy and related cognitive impairments may open doors in the development of novel therapeutical approaches for this disorder.